若向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$满足 $\left|\stackrel{\rightharpoonup }{a}\right|=1,\left|\stackrel{\rightharpoonup }{b}\right|=2$,且 $\stackrel{\rightharpoonup }{a}$与 $\stackrel{\rightharpoonup }{b}$的夹角为,则 $\left|\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}\right|=$.